1. Field of the Invention
The invention relates to the isolation and functional characterisation of a novel mammalian Su(var)3-9 homologue, Suv39h2, and its use.
2. Related Art
In eukaryotes, control of gene expression and the functional organisation of chromosomes depends on higher-order chromatin (Paro and Harte, 1996; Karpen and Allshire, 1997). In addition to its role in somatic cells, higher-order chromatin is also involved in chromosomal dynamics during meiosis (Dernburg et al., 1996). Although condensation and pairing of meiotic chromosomes is evolutionarily highly conserved, meiosis in male mammals is exceptional because the heteromorphic X and Y chromosomes undergo facultative heterochromatinisation that is accompanied by transcriptional silencing (Handel and Hunt, 1992). This selective inactivation of the male sex chromosomes, which is cytologically defined by the appearance of the so-called XY body or sex vesicle (Solari, 1974), has been proposed to restrict promiscuous pairing or recombination between nonhomologous chromosomes, thereby reducing the risk for aneuploidy (Handel and Hunt, 1992). In fact, failure to form this specialised chromatin structure in the XY body prevents successful spermatogenesis (Kot and Handel, 1990; Matsuda et al., 1991).
Su(var) genes were initially identified by genetic screens on centromeric position effects in Drosophila melanogaster (Reuter and Spierer, 1992) and Schizosaccharomyces pombe (Allshire et al., 1995). Since Su(var) genes suppress position effect variegation (PEV), their gene products have been implicated in the organisation of repressive chromatin domains (Henikoff, 1997). Indeed, isolated family members encode either chromosomal proteins or enzymes that can modify chromatin (Wallrath, 1998).
Drosophila Su(var)3-9 and its S.pombe clr4 homologue are the only modifying loci whose gene products combine the characteristic chromo and SET domains. Whereas the 60 amino acid chromo domain (Paro and Hogness, 1991; Aasland and Stewart, 1995; Koonin et al., 1995) represents a protein-specific interaction surface (Messmer et al., 1992; Platero et al., 1995) that resembles an ancient histone-like fold (Ball et al., 1997), the structure of the 130 amino acid SET domain (Jenuwein et al., 1998) is currently undefined. However, it has recently been shown that the SET domain of Suv39h1 harbours an intrinsic histone methyltransferase HMTase activity, which is specific for lysine 9 of histone H3 (Rea et al., 2000). These data suggest that Suv39h homologues exert their function through the organisation chromatin structure via histone H3 methylation.
The corresponding mouse (Suv39h1) and human (SUV39H1) Su(var)3-9 homologues have been identified and it has been demonstrated that SUV39H1 represents a functional mammalian homologue of Su(var)3-9 in transgenic flies (Aagaard et al., 1999). Immunolocalisation of endogenous Suv39h1 or SUV39H1 proteins in mammalian cells indicated enriched distribution at heterochromatic foci during interphase and transient accumulation at centromeric positions during mitosis (Aagaard et al., 2000). In addition, Suv39h1 or SUV39H1 associate with M31 (HP1β), one mammalian homologue of Drosophila HP1, indicating the existence of a mammalian SU(VAR) protein complex(es) (Aagaard et al., 1999). Moreover, deregulated SUV39H1 can induce ectopic heterochromatin and redistribute endogenous M31 (HP1β) (Melcher et al., 2000). These data defined Suv39h1 or SUV39H1 as novel heterochromatic HMTase proteins that are involved in the structural organisation of mammalian higher-order chromatin in somatic cells.